Magnetic recording media, inclusive of disks and memory tapes, used for recording audio, video, computer or other information generally consist of a non-magnetic base support, a magnetic coating layer disposed on one side of the support and containing ferromagnetic fine particles dispersed in a binder, and a back coat layer disposed on the other side of the support and containing a non-magnetic fine powder dispersed in a binder.
In recent years, these magnetic recording media have been required to provide high density recording. Signal-to-noise ratio elevation and noise reduction have been achieved by using finer ferromagnetic powders, using alloy powders, employing high loading factor values or finishing the magnetic recording medium surface to extreme smoothness, for instance. For achieving high density recording in a certain given time, it is necessary to increase the rate of writing or transfer of information into magnetic recording media and, therefore, high magnetic recording medium speeds are required. For attaining such high travel speeds, it is essential that the magnetic recording media should be good in running, antistatic and head cleaning properties. For this purpose, fillers such as carbon black and abrasives having a Mohs hardness of not less than 8 are used in the magnetic layer, as disclosed, for example, in JP-A-59-193533, JP-A-59-186125, JP-A-59-191133, JP-A-58-189831 and JP-A-59-63029 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Furthermore, carbon black is used in the back coat layer to improve running and durability, as disclosed in JP-A-52-96505, JP-A-55-28507, JP-B-54-21248 and JP-B-52-17401 (the term "JP-B" as used herein means an "examined Japanese patent publication).
Furthermore, whether these magnetic recording media can store magnetic records stably over scores of years is another important problem. Approaches to solving the storage stability problem for magnetic recording media may be summarized as follows. First, changes in magnetic characteristics of ferromagnetic fine powders during storage should be suppressed or inhibited. Thus, deterioration of magnetic characteristics due to oxidation in alloy powders or due to changes in distribution of Co or Fe.sup.++ in particles of Co-containing .gamma.-Fe.sub.2 O.sub.3 should be prevented. Second, deterioration of binders should be prevented. It is known that binder coat layers are generally subject to deterioration due to ultraviolet radiation, temperature and/or humidity, as disclosed in E. F. Cuddhy, IEEE Trans. Mag., Mag. 16, 4, 558 (July 1980). Vinyl chloride-vinyl acetate copolymer resins and cellulosic resins are known to be less subject to such deterioration than polyurethane resins and are used in an amount of about 10 to 60% by weight on the whole binder basis. Third, foreign matter formation in magnetic recording media, which may cause dropouts or induce head gap clogging leading to spacing losses, thereby hindering regeneration of magnetic records, should be prevented. A variety of causes have been known for such foreign matter formation and have been eliminated, including, for example, certain fatty acids, certain fatty acid esters, oligomers in macromolecular supports, and certain inorganic salts.
As for the inorganic salt formation, it is known that the metal ion content or counter anion content of ferromagnetic fine powders in the magnetic layer should be reduced (cf. JP-B-40-11733, JP-B-48-27118, JP-A-56-51029 and JP-A-56-101649) or that the Na content of carbon black in the back coat layer should be suppressed (cf. Japanese Patent Application No. 63-97380). It is also known that the content of water-soluble salts coming from ferromagnetic metal-forming metals (e.g., Fe, Co, Ni) and/or other additive metals (e.g., Cr, Mn, Zn) should be suppressed (cf. JP-A-56-51029). It is known that ferromagnetic fine powders have a Ca content of about 100 ppm (parts per million) and that carbon black has an Na content of 170 to 500 ppm. Furthermore, ferromagnetic metal fine powders are known to have a water-soluble Fe content of 80 to 2,900 ppm.
Even by the efforts mentioned above, however, it has been very difficult to prevent dropout and head clogging due to formation of inorganic salts. This is considered to be due to the difficulty in specifying the sources of such inorganic salts.